Bismaleimide resins are important resins in commerce for various uses, including fiber reinforced composite and structural adhesives. However, bismaleimide resins, while possessing high strength at elevated temperatures, tend to be somewhat brittle. Accordingly, it is common to modify the bismaleimides with additional comonomers to improve impact resistance.
Curable mixtures based on aromatic bismaleimides and propenyl compounds are known, for example, from H. D. Stenzenberger et al. ("Stenzenberger"), relating to various curable resins from bismaleimide and alkenyl phenyl hydroxy ether, and laminated articles produced from said resin as disclosed in U.S. Pat. Nos. 4,871,821, 4,917,954, 4,789,704, 4,939,305 and 5,120,824. The combination of a bismaleimide and a propenyl compound by slurry mixing Is not disclosed in any of the Stenzenberger patents.
Certain curable mixtures containing a bismaleimide and a propenyl compound are disclosed In A. Kramer, U.S. Pat. No. 5,013,804 ("Kramer"). Although various aromatic bismaleimides are contemplated, no disclosure, either as teaching or suggestion using methylenedianiline bismaleimide, is provided. Additionally, Kramer discloses a fusion process for bringing together a bismaleimide compound and a propenyl compound. Kramer does not teach or suggest the thermosetting polymer made, and the making thereof, using slurry mixing.
The process of slurry mixing bismaleimide resins is disclosed in L. Repecka, U.S. Pat. No. 5,003,018 ("Repecka"). Certain aromatic bismaleimides in combination with liquid co-reactants, like o,o'-diallylbisphenol A, are disclosed. All of the above mentioned patents are incorporated herein by reference.
Many comonomers are physically incompatible with the bismaleimide resins because following initial formulation at elevated temperatures, the bismaleimide component may crystallize during processing. The large crystals which then form result in a resin system which is heterogeneous, difficult to process and which may be prone to microcracking when used to prepare carbon fiber reinforced composites. In such a resin system, wherein the bismaleimide components are physically incompatible, adhesives and prepregs prepared therefrom by the conventionally prepared systems often do not have the drape and tack desired of them.
As a result, there is a need in this technical area for a bismaleimide resin that has a high temperature capability, superior oxidative stability, and acceptable tack and drape to make composite parts.